1. Field of the Invention
The present invention relates to an image forming apparatus adapted for use for example in a copier.
2. Description of the Prior Art
In an example of the copying process for a copier wherein the present invention is applicable as disclosed in the U.S. Pat. Nos. 3,666,363 and 4,071,361, the surface of a photosensitive drum provided, with a photosensitive element consisting of an electroconductive layer, a photoconductive layer and an insulating layer, is subjected to a uniform precharging (for example positive charging) by means of a primary charger along with the rotation of said drum, and subjected to a scanning exposure of a light image in synchronization with the displacement of an original carriage (or an optical system) simultaneously with a charge elimination by means of a recharger of an alternating current (or a direct current of a polarity opposite to that of said primary charger) thereby to form an electrostatic latent image corresponding to said light image. Said latent image is enhanced by a flash or whole-surface exposure to a higher contrast and is rendered visible in a developing station by a developer principally consisting of toner particles. The visible image thus obtained is transferred by a corona discharge of a polarity which is the same as that of said toner (namely negative if precharging is positive) onto a transfer sheet consisting of plain paper and fixed thereon by means of a heater during transportation. On the other hand the developer particles remaining on the surface of said photosensitive drum after said transfer are removed by a cleaning blade while the retentive charge on said surface is removed by a lamp and a corona discharger to allow repetitive use of the photosensitive element. Copies of a desired number are obtained by repeating the copying process as described above.
In such process, even though the photosensitive element is cleaned before reuse, it frequently happens that the surface of photosensitive element becomes smeared by various causes for example toner deposition when the machine is let to stand without use, such smearing being particularly marked in case of liquid development. Consequently the first image obtained after the restart of a machine often appears unacceptable. Also such trouble may result from uneven potential on the surface of photosensitive element when the machine is restarted.
Also in case the same portion on the surface of photosensitive element is subjected to repeated processing, there may result a local accumulation of toner or an uneven potential to deteriorate the image quality. In addition to such limited reliability of image quality, the high-speed performance and precision of process operations in the sequence control is also associated with a limited reliability. Namely in the conventional processes the control of the operation loads required for the process control has been conducted on the basis of particular surface positions of a recording element such as the photosensitive element. Consequently the control timing may become limited by the dimensions of the recording element, eventually leading to an unnecessary time loss. Also there has been required an additional circuitry in order to achieve proper timing operations according to the desired copy sizes without unnecessary functions of the process means.
Furthermore the transistor-transistor-logic circuitry which has been commonly employed for control has only a small noise margin (proportion of noise acceptable in the signal), thus being extremely sensitive to noise, particularly in copiers involving the use of high voltages. The resulting frequent use of RC filters (filters consisting of resistors and condensers) for preventing noise has inevitably resulted in an increased number of parts with a complex circuit structure. Consequently it may often result that a modification of sequence control is not achievable despite the use of complicated logic processing.
On the other hand the machines utilizing microcomputers have become known in recent years. However there has not been known a solution for uncertain factors such as operation errors and for limitations in the machine performance in case of applying such computers to such image forming apparatus.
Furthermore the detection of paper jamming resulting from defective paper feeding, required to identify different paper sizes and to distinguish single or multiple copying, requires a complicated circuit composition and results in an undesirably low precision of detection. Also an operation error in the detection of paper jamming or paper feeding is fatal in such continuous copying apparatus, and much time has been required for designing and investigation for preventing such trouble.
Furthermore, in the operation confirmation test or heat running test without paper feeding during the course of maintenance or assembly of the machine, it becomes necessary to disable the circuit for detecting an improper state, such as jamming detection circuit or the circuit for detecting no paper, and therefore the operation has been cumbersome because of the independent structure of said circuits. Also in case of the transistor-transistor-logic control explained above, various timers which are indispensable for the control of copier have to be composed to separate circuits, which are expensive particularly in case of timers of a longer period.